Exec=/usr/bin/shred -uzn 2 %F; two overwrites, plus a final 'zero' overwrite is probably overkill on modern high-density drives.

Selection=any so that more than one file can be selected for shredding.

EscapeSpaces=true I added this because I was getting errors on files with spaces in the filename.

For SSDs or thumb drives, or HDDs with filesystem ext3/4 with journaling enabled, then shred is worthless. The only way to securely erase these devices is by using dd on the entire drive. And that may not even be enough. Smashing the device with a sledge hammer will also make the data unavailable, thus effectively erasing the medium.

If you also want a custom icon to go along with your custom action, then the nemo source code appears to just be passing the icon name to a GTK api function and letting it do the work of resolving the name to a file (apologies in advance if I am incorrect on this.. I only skimmed the code, it's been years since I played around in C/C++, and I am unfamiliar with using the GTK framework).

For the above, the most likely png was '/usr/share/pixmaps/antimicro.png'. I saw some of the other icons in other formats (xpm, svg). Not sure about jpg/gif/ico/etc (i had image magick installed so just did 'convert file.ico file.png'). I assume square icon sizes are required. Anyway, this had worked for me: